1. Field of the Invention
This invention relates to a quadrature detector used in FM demodulation and, more particularly, a quadrature detector of a simpler structure which also requires a very low power supply voltage.
2. Description of the Prior Art
In the past, double-balanced mixers with frequency dependent phase-shift networks have been used extensively in implementing quadrature demodulation of FM signals. A conventional, active, double-balanced mixer, such as the one illustrated on pages 103-106 of Analog Integrated Circuits, by J. A. Connelly, published by John Wiley and Sons, requires a power supply voltage in the range of two volts or higher. In order to implement quadrature demodulation with a lower power supply voltage of about one volt, a quadrature detector of the type illustrated in FIG. 1 has been proposed. FIG. 2 depicts various waveforms which illustrate the operation of the detector shown in FIG. 1. As shown in FIGS. 1 and 2, the signal from the FM receiver's limiter (not shown), V.sub.1, is applied to one input of a gating circuit 11; another input, namely V.sub.2, which is a phase-shifted by a certain amount, .0., and limited version of V.sub.1, is obtained by processing V.sub.1 through a phase shifter 12 and a limiter 13, is applied to the other input of the gating circuit; the amount of phase-shift is a function of the frequency of V.sub.1. The output of the gating circuit, V.sub.3, is a pulse-train whose duty-cycle varies with the frequency of V.sub.1. This pulse-train is then integrated with a low-pass filter (LPF) 14 in order to obtain the original modulating signal.
Note that the prior art demodulator 15 shown in FIG. 1 requires limiter 13 between the phase-shift network output and the gating circuit input. This is because the output of the phase-shift network is sinusoidal and the additional limiter is used to mark or sharpen the zero-crossings of the sinusoid so that the gating circuit switches at the proper time intervals.